Polysiloxane resins are polymers having advantages including high transparency, heat resistance, and low cost and ease of working as compared with quartz glass and thus considered attractive as a material for forming optical transmission lines such as optical waveguides. However, when thick films are formed from polysiloxane resins, they give rise to several problems including poor crack resistance, a transmission loss and degradation in a humid environment, and cumbersome steps involved in thick film formation.
To solve these problems of polysiloxane resins, JP-A 2001-59918 discloses a technique of preparing an optical waveguide by forming a thick film of a silicone ladder resin to a thickness in excess of 20 μm. The resin used is a thermosetting resin, and the optical waveguide preparing process involves a dry etching step which is cumbersome.
Japanese Patent No. 3,133,039 discloses a technique of directly preparing a core ridge of an optical waveguide simply by forming a coating of a photo-curable silicone oligomer, exposing the coating to light through a mask for effecting photo-crosslinking, and washing away the masked or unreacted portion of the coating with a suitable solvent. This optical waveguide-forming material is characterized by a siloxane oligomer with which a molecule having a photo-polymerizable group is admixed, but not an oligomer having photo-polymerizable groups directly bonded to silicon atoms through covalent bonds.
As a general rule, plastic based optical materials including polysiloxanes experience a propagation loss of light by moisture absorption in a humid environment. JP-A 2001-33640 solves this problem by covering a clad layer of an optical waveguide with an amorphous fluororesin layer. This does not impart moisture resistance to the resins of which the core and clad layers are formed. JP-A 08-311139 corresponding to U.S. Pat. No. 5,738,976 discloses a photo-curable organopolysiloxane composition comprising an acrylic functional organopolysiloxane resin which does not possess any functional group such as alkoxy or hydroxyl group in the molecule, that is, is terminated with a triorganosiloxy group, and is thus inferior in adhesion to the substrate or the like under heating conditions. No reference is made to the refractive index which is an important factor as the optical waveguide-forming material.
Therefore, there is a need to have a technique capable of fabricating in a simple and commercially advantageous manner an optical waveguide of quality as an optical transmission line, having a thick-film coating which is improved in heat resistance, moisture resistance, and adhesion to substrate.